congratulations on the decisioin to switch!
Also don't rule out 1460s or even 1480s. 60s & 80s have lots heavier drive train (final drives and transmissions) than the 40s did. Depending on what part of the country you are in, I'm in central Iowa and you can get a "useable" 1400 series for $5000 or less. Whether its a 40-60-80 around here, condition trumps model when it comes to price. Of course one in better shape with more options (rock trap, chopper, etc.) will cost you more. The heads will cost you about the same or more than what you will probably pay for the combine. Note that the 1400s had electric over hydraulics starting in 1981 i believe, so if you're looking at a 1020 head I think you have to find a combine with electric controls. The pre 81 combines were mechanical and I'm not sure if a 1020 head can be made to work in auto-height with a mechanical cable or not.
As far as the 1600 series goes, yes they are more money, (seems they bring about $10k more) but with all these machines they are all getting old now and wiring and hydraulic leaks will be your biggest worry. Every machine you look at pull the side panel off the right side of the cab and look for mouse damage and general condition of the wiring. With the 1600s all you are getting over the 14s is a little bigger concave area and bigger clean grain elevator. For what you are doing I don't think that's worth 10 grand.
Any of them will work fine for what you are doing. I would shop by conditon & price instead of a particular model. Steel parts are easy to patch/replace but when you start chasing bad wiring and hard/rotton hydraulic hoses & orings you can pull your hair out in a hurry.
I would think that $5000 per unit, $15000 total would set you up with a pretty useable package, for a 1400 series and both heads.
Happy axial-flow hunting!